The invention relates to transducers for measuring changes in environmental pressures and, in particular, to transducers utilizing capacitance change principles to achieve the measurements.
As already indicated, the present invention broadly contemplates a transducer having a diaphragm exposed to pressure variations in its environment. The resulting flex of the diaphragm moveably varies the plate spacing of the capacitor arrangement to produce a capacitance-modulated output signal directly proportional to the sensed pressure variations. Such a transducer obviously is capable of providing operable pressure sensing devices. However, probably because of particular difficulties to be considered, transducers of this general type apparently have not been developed to any appreciable extent. Certainly, as far as presently is known, there are no prior art developments capable of being used for the particular purposes contemplated in the present invention.
As to the presently-contemplated uses, the present transducer primarily is intended to measure pressure changes within a solid material. Specifically it is intended to be implanted or imbedded inside a solid propellent charge within a rocket motor. As is known, the condition of the propellant charge must be continuously monitored to provide a propellant stress analysis and to serve other functions such as the detection of propellant gasing, the determination of propellant cure time and the detection of propellant cracks and case bond failures. Also, monitoring must be conducted for exceedingly long periods of time and this requirement itself imposes difficulties which cannot be resolved in conventional manners.
Conventional capacitive transducers are not suited for use in the particular applications with which the present invention is concerned. For example, such transducers normally employ power and signal cables. However, the physical presence of such cables in the solid material provides undesirable paths for temperature dissipation and pressure leakage and they also produce additional stress perturbation. The use of batteries avoids the cable problem but batteries have a limited life and also they present safety problems.
Another very real difficulty is to assure the accuracy and reliability of the transducers. For example, accuracy in pressure variation measurements requires an unvarying long-term stability and, if such stability is to be achieved with an acceptable degree of certainty, it is necessary to provide a relatively high initial capacitance and a large full scale capacitance change with pressure. Further, a particularly vital consideration is that the transducers demonstrate a linear capability or, in other words, a signal output which is linearly proportional to the applied stress and, along with the linearity, the transducers should yield a high output signal. Although the relationship between linearity and high output will be discussed subsequently it can be noted at this point that high output is to some extent a function of the degree to which the capacitor plates are moved by the pressure-sensing diaphragm, i.e., large excursions, can produce comparably large changes in capacitance. However, as is known, when capacitor plates are moved into a relatively close proximity one with the other, the capacitance begins to vary in a non-linear or exponential manner and this undesirable result must be controlled by limiting the plate movement to regions in which the capacitance changes are linear. Consequently, the limited movement needed for linearity conflicts somewhat with the desire for high output levels. Apparently, factors such as these have presented real difficulties which have discouraged the development of sensors of this type.
Another problem involves the fact that these instruments may be implanted or used in materials which are subject to widely-varying temperatures. If so, temperature-produced dimensional changes may result in an undesirable loading or unloading effect on the diaphragm. Consequently, any sensors or transducers in which accuracy over relatively long periods of time is significant must take into consideration the temperature effects and, consequently, must provide some means for temperature compensation.
The objects of the present invention are to provide a transducer which, generally, resolves problems which have been mentioned in a satisfactory manner. In particular, an object is to provide a pressure-sensing transducer having a relatively high output which also provides a linear representation of the stresses acting on the transducer diaphragm.
Another object is to assure the long term stability of the transducer and, in conjunction with the stability, to avoid the degrading effects of dimensional changes produced by widely-varying temperatures.
A further object which has briefly been considered is to provide a capacitive-type of transducer adapted to be linked to a telemetry system both for input power and output data transmittal.
Other objects are to provide a transducer arrangement which is relatively simple, rugged and sufficiently reliable to allow operation over extended periods of time.
In general, the objects of the invention are achieved by the use of the so-called load piston which is carried centrally by the diaphragm in firm contact with the capacitor plates and which is a sufficient size to apply the relatively large excursions or movements derived from the central or axial portion of the diaphragm over the entire capacitor plate surface. Plate movement then can be closely controlled so that the capacitance variations are within the linear region of the plate spacing. Also, the load piston can be tailored to provide the desired temperature compensation or, in other words, to counteract the temperature-produced dimensional changes occuring in other parts of the transducer. In the preferred form, a stack of capacitors is employed and various dimensions, including the plate spacing and the diaphragm thickness, as well as the load piston thickness, are controlled to assure both the desired high output and the linearity of the output. Other features of the invention, will be considered in the detailed description of the invention.